In recent years, a high-strength steel sheet or light metal such as Al alloy has been applied to vehicle members for the purpose of weight decrease for improving vehicle fuel efficiency and the like. The light metal such as Al alloy has an advantage in that specific strength is high; however, it is much more expensive than a steel, and therefore, the application of the light metal is limited to special uses. Accordingly, it is necessary to realize a steel sheet having higher strength in order to promote weight decrease of vehicles over a wide range and at a lower cost.
In general, the increase in strength of a material causes material characteristics such as formability (workability) to deteriorate. Therefore, it is important to achieve the increase in strength without the deterioration in the material characteristics for developing a high-strength steel sheet. Particularly, as characteristics which are required for steel sheets used for inner plate members, structural members and underbody members, stretch-flange formability, ductility, fatigue durability, in particular, fatigue durability after a hole making because of frequent hole making (piercing), corrosion resistance and the like are important. It is important to balance the high strength and these characteristics at high level.
Transformation induced plasticity (TRIP) steel is disclosed in which both of an increase in strength and excellent various characteristics, particularly, formability are realized (for example, see Patent Documents 1 and 2). The TRIP steel includes residual austenite in the microstructure of the steel; and thereby, a TRIP phenomenon is expressed during a forming process. Accordingly, formability (ductility and deep drawability) is dramatically improved. However, stretch-flange formability generally deteriorates. Accordingly, a steel sheet having high strength and remarkably excellent stretch-flange formability is desired.
Several hot-rolled steel sheets having excellent stretch-flange formability are disclosed. Patent Document 3 discloses a hot-rolled steel sheet having a single phase microstructure of acicular ferrite. However, in the microstructure of a single low-temperature transformation product, ductility is low, and it is difficult to utilize the steel sheet for uses other than stretch-flange forming.
Patent Document 4 discloses a steel sheet having a microstructure consisting of ferrite and bainite. In the steel having a composite microstructure, relatively excellent ductility is obtained; however, a hole expansion ratio which is an index indicating stretch-flange formability tends to be low.
In addition, Patent Document 5 discloses a steel sheet having a high volume fraction of ferrite. However, since the steel sheet contains a large amount of Si, a problem is caused in fatigue properties and the like in some cases. In order to avoid a negative effect caused by Si, it is necessary to perform surface modification during and/or after a hot rolling. Therefore, there are many problems in that the introduction of special facilities is required or the productivity deteriorates.
Patent Documents 6 and 7 disclose a hot-rolled steel sheet in which Ti is added and which is excellent in hole expansionability. However, Ti/C is not properly controlled and a hole expansion ratio is not very high.
[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2000-169935
[Patent Document 2] Japanese Unexamined Patent Application, Publication No. 2000-169936
[Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2000-144259
[Patent Document 4] Japanese Unexamined Patent Application, First Publication No. S61-130454
[Patent Document 5] Japanese Unexamined Patent Application, First Publication No. H08-269617
[Patent Document 6] Japanese Unexamined Patent Application, First Publication No. 2005-248240
[Patent Document 7] Japanese Unexamined Patent Application, First Publication No. 2004-131802